1. Field of the Invention
The present invention relates to a method for efficiently setting transmit power for each user in a system supporting spatial multiplexing using multiple Transmission (Tx) and Reception (Rx) antennas for data transmission between a Base Station (BS) and multiple users.
2. Discussion of the Related Art
Beyond the conventional use of a single Tx antenna and a single Rx antenna, Multi-Input Multi-Output (MIMO) is a technology for increasing the transmission and reception efficiency of data by use of a plurality of Tx antennas and a plurality of Rx antennas. That is, MIMO is a technology for increasing capacity and improving performance by use of multiple antennas at both a transmitter and a receiver in a wireless communication system. ‘MIMO’ may be used interchangeably with ‘multi-antenna’.
The MIMO technology does not depend on a single antenna path to receive a whole message. Rather, it completes the data by combining data fragments received through a plurality of antennas. With the multi-antenna technology, data rate may be increased within a system coverage size, or system coverage may be extended with a specific data rate ensured. Furthermore, MIMO is a future-generation mobile communication technology that may find its use in a wide range including mobile terminals, relays, etc. MIMO may overcome mobile transmission capacity problems encountered with the increased data communications. Considering the future-generation mobile communication requires a much higher data rate than legacy mobile communication, it is expected that an efficient MIMO scheme is needed.
Among a variety of technologies for increasing transmission efficiency under study, MIMO using a plurality of antennas at both a transmitter and a receiver is currently attracting much interest as promising to remarkably increase communication capacity and transmission/reception performance without additional frequency allocation or power increase.
As illustrated in FIG. 1, compared to the use of a plurality of antennas at only one of a transmit end and a receive end, the use of an increased number of antennas at both the transmitter and the receiver increases a theoretical transmission capacity in proportion to the number of antennas, thereby increasing frequency efficiency significantly. Since the theoretical capacity increase of the MIMO system was proved in the middle 1990's, many techniques have been actively studied to increase data rate in real implementation. Some of the techniques have already been reflected in various wireless communication standards for 3rd Generation (3G) mobile communications, future-generation Wireless Local Area Network (WLAN), etc.
Active studies are underway in many respects regarding the MIMO technology, inclusive of studies of information theory related to calculation of MIMO communication capacity in diverse channel environments and multiple access environments, studies of measuring radio channels and deriving a model for a MIMO system, studies of time-space signal processing techniques to increase transmission reliability and transmission rate, etc.
There are two types of MIMO schemes: spatial diversity and spatial multiplexing. Spatial diversity increases transmission reliability using symbols that have passed in multiple channel paths, whereas spatial multiplexing increases transmission rate by transmitting a plurality of data symbols simultaneously through a plurality of Tx antennas. Taking the advantages of these two schemes is a recent active study area.
FIG. 1 illustrates the configuration of a typical MIMO communication system.
As illustrated in FIG. 1, it is assumed that there are NT Tx antennas and NR Rx antennas in the typical MIMO communication system. For the convenience's sake of description, the MIMO system will be modeled as a mathematical model described below.
Regarding a transmission signal, if NT Tx antennas are used, up to NT symbols may be transmitted at one symbol timing. If the number of actual transmission information symbols be denoted by NL, the NL transmission information symbols are collectively expressed as the following vector. Herein, it is defined that the NL transmission information symbols are transmitted through different virtual layers.s=[s1, s2, . . . , sNL]T  [Equation 1]where each of the transmission information symbols s1, s2, . . . , sNL may be transmitted at a different transmit power level. If the root values of the transmit power levels of the transmission information symbols are denoted by P1′, P2′, . . . , PNL′, respectively, the transmit power-controlled transmission information symbols ŝ may be given vector as [Equation 2].ŝ=[ŝ1, ŝ2, . . . , ŝNL]T=[P1′s1, P2′s2, . . . , PNL′sNL]T  [Equation 2]
Also, ŝ may be expressed as a diagonal matrix P of the transmit power levels.
                              s          ^                =                                            [                                                                                          P                      1                      ′                                                                                                                                                                                                                                                                                                      0                      ⁢                                                                                                                                                                                                                                                                                                        P                      2                      ′                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    ⋱                                                                                                                                                                                          0                                                                                                                                                                                                                                                                                  P                                              N                        L                                            ′                                                                                  ]                        ⁡                          [                                                                                          s                      1                                                                                                                                  s                      2                                                                                                            ⋮                                                                                                              s                                              N                        L                                                                                                        ]                                =          Ps                                    [                  Equation          ⁢                                          ⁢          3                ]            
Meanwhile, NT transmission signals x1, x2, . . . , xNT transmitted through actual each antennas may be configured by applying a weight matrix W to the transmit power-controlled information vector ŝ. The signal vector x is represented as follows. Herein, wij denotes a weight between an ith Tx antenna and a jth layer and the weight are expressed as the matrix W. W is referred to as a weight matrix or a precoding matrix.
                                                        x              =                              [                                                                                                    x                        1                                                                                                                                                x                        2                                                                                                                        ⋮                                                                                                                          x                        i                                                                                                                        ⋮                                                                                                                          x                                                  N                          T                                                                                                                    ]                                                                                        =                                                [                                                                                                              w                          11                                                                                                                      w                          12                                                                                            …                                                                                              w                                                      1                            ⁢                                                                                                                  ⁢                                                          N                              L                                                                                                                                                                                                                    w                          21                                                                                                                      w                          22                                                                                            …                                                                                              w                                                      2                            ⁢                                                                                                                  ⁢                                                          N                              L                                                                                                                                                                                          ⋮                                                                                                                                                                                          ⋱                                                                                                                                                                                                                                                            w                                                      i                            ⁢                                                                                                                  ⁢                            1                                                                                                                                                w                                                      i                            ⁢                                                                                                                  ⁢                            2                                                                                                                      …                                                                                              w                                                      iN                            L                                                                                                                                                              ⋮                                                                                                                                                                                          ⋱                                                                                                                                                                                                                                                            w                                                                                    N                              T                                                        ⁢                            1                                                                                                                                                w                                                                                    N                              T                                                        ⁢                            2                                                                                                                      …                                                                                              w                                                                                    N                              T                                                        ⁢                                                          N                              L                                                                                                                                                            ]                                ⁡                                  [                                                                                                                                          s                            ^                                                    1                                                                                                                                                                                          s                            ^                                                    2                                                                                                                                    ⋮                                                                                                                                                                  s                            ^                                                    j                                                                                                                                    ⋮                                                                                                                                                                  s                            ^                                                                                N                            L                                                                                                                                ]                                                                                                        =                              W                ⁢                                  s                  ^                                                                                                        =              WPs                                                          [                  Equation          ⁢                                          ⁢          4                ]            
The above-mentioned transmission signal x may be considered in two cases: spatial diversity and spatial multiplexing. In spatial multiplexing, different signals are multiplexed to different layers prior to transmission. Accordingly, the elements of the information vector s have different values. In contrast, the same signal is transmitted in a plurality of channel paths in spatial diversity. As a result, the elements of the information vector s have the same value. Spatial multiplexing and spatial diversity may be used in combination. For example, the same signal may be transmitted through three layers in spatial diversity, while different signals may be transmitted through the other layers in spatial multiplexing.
For NR Rx antennas, signals received at the Rx antennas, y1, y2, . . . , yNR may be represented as the following vector.y=[y1, y2, . . . , yNR]T  [Equation 5]
Channels are identified according to the indexes of Tx and Rx antennas and the channel between a jth Tx antenna and an ith Rx antenna is represented as hij. It is to be noted herein that the index of the Rx antenna precedes that of the Tx antenna in hij The channels may be represented as a vector and a matrix by grouping them. For instance, the channels from the total of NT Tx antennas and an ith Rx antenna may be given ashiT=[hi1, hi2, . . . , hiNT]T  [Equation 6]
FIG. 2 illustrates the channels from the NT Tx antennas to the ith Rx antenna.
Also, the channels from the NT Tx antennas to the NR Rx antenna may be expressed as the following matrix.
                                                        H              =                              [                                                                                                    h                        1                        T                                                                                                                                                h                        2                        T                                                                                                                        ⋮                                                                                                                          h                        i                        T                                                                                                                        ⋮                                                                                                                          h                                                  N                          R                                                T                                                                                            ]                                                                                        =                              [                                                                                                    h                        11                                                                                                            h                        12                                                                                    …                                                                                      h                                                  1                          ⁢                                                                                                          ⁢                                                      N                            T                                                                                                                                                                                                  h                        21                                                                                                            h                        22                                                                                    …                                                                                      h                                                  2                          ⁢                                                                                                          ⁢                                                      N                            T                                                                                                                                                                          ⋮                                                                                                                                                                          ⋱                                                                                                                                                                                                                                      h                                                  i                          ⁢                                                                                                          ⁢                          1                                                                                                                                    h                                                  i                          ⁢                                                                                                          ⁢                          2                                                                                                            …                                                                                      h                                                  i                          ⁢                                                                                                          ⁢                                                      N                            T                                                                                                                                                                          ⋮                                                                                                                                                                          ⋱                                                                                                                                                                                                                                      h                                                                              N                            R                                                    ⁢                          1                                                                                                                                    h                                                                              N                            R                                                    ⁢                          2                                                                                                            …                                                                                      h                                                                              N                            R                                                    ⁢                                                      N                            T                                                                                                                                              ]                                                                        [                  Equation          ⁢                                          ⁢          7                ]            
Actual channels experience the above channel matrix H and then are added with Additive White Gaussian Noise (AWGN). The AWGN n1, n2, . . . , nNR added to the NR Rx antennas is expressed as the following vector.n=[n1, n2, . . . , nNR]T  [Equation 8]
From the above modeled equations, the received signal is given as
                                                        y              =                              [                                                                                                    y                        1                                                                                                                                                y                        2                                                                                                                        ⋮                                                                                                                          y                        i                                                                                                                        ⋮                                                                                                                          y                                                  N                          R                                                                                                                    ]                                                                                        =                                                                    [                                                                                                                        h                            11                                                                                                                                h                            12                                                                                                    …                                                                                                      h                                                          1                              ⁢                                                                                                                          ⁢                                                              N                                T                                                                                                                                                                                                                                      h                            21                                                                                                                                h                            22                                                                                                    …                                                                                                      h                                                          2                              ⁢                                                                                                                          ⁢                                                              N                                T                                                                                                                                                                                                          ⋮                                                                                                                                                                                                          ⋱                                                                                                                                                                                                                                                                                  h                                                          i                              ⁢                                                                                                                          ⁢                              1                                                                                                                                                            h                                                          i                              ⁢                                                                                                                          ⁢                              2                                                                                                                                …                                                                                                      h                                                          i                              ⁢                                                                                                                          ⁢                                                              N                                T                                                                                                                                                                                                          ⋮                                                                                                                                                                                                          ⋱                                                                                                                                                                                                                                                                                  h                                                                                          N                                R                                                            ⁢                              1                                                                                                                                                            h                                                                                          N                                R                                                            ⁢                              2                                                                                                                                …                                                                                                      h                                                                                          N                                R                                                            ⁢                                                              N                                T                                                                                                                                                                          ]                                    ⁡                                      [                                                                                                                        x                            1                                                                                                                                                                            x                            2                                                                                                                                                ⋮                                                                                                                                                  x                            j                                                                                                                                                ⋮                                                                                                                                                  x                                                          N                              T                                                                                                                                            ]                                                  +                                  [                                                                                                              n                          1                                                                                                                                                              n                          2                                                                                                                                    ⋮                                                                                                                                      n                          i                                                                                                                                    ⋮                                                                                                                                      n                                                      N                            R                                                                                                                                ]                                                                                                        =                              Hx                +                n                                                                        [                  Equation          ⁢                                          ⁢          9                ]            
The numbers of users and the numbers of antennas at the transmitter and the receiver may be combined in many ways. As a kind of spatial multiplexing, when a transmitter transmits signals to a plurality of users, this is called multi-user MIMO. The multi-user MIMO will be considered separately on a downlink (forward link) and on an uplink (reverse link). The multi-user MIMO downlink is a transmission direction from a BS to a plurality of UEs, whereas the multi-user MIMO uplink is a transmission direction from a plurality of UEs to a BS.
In two extreme downlink cases, for example, signals may be transmitted to a single user through NL layers, and to NL users, through one layer for each user. An intermediate case may also be contemplated in which one layer is used for a user and three layers for another user, for example. Likewise on the uplink, there may be two extreme cases in which one user transmits signals through a total of NL layers and NL users transmit signals, each user through one layer. Also, an intermediate uplink case is available, in which a user uses one layer and another user uses three layers, for example.
Conventionally, when a BS transmits a transmit power ratio value to a UE, a large overhead results.